A Novel Bone Conduction Implant System

Licentiate thesis, 2012

Bone conduction is the process by which an acoustic signal vibrates the skull bones to stimulate the cochlea. Patients with pure conductive hearing loss, mixed hearing loss, and single sided deafness are sometimes poorly rehabilitated by conventional air conduction hearing aids due to for example the functionality losses in the middle ear. With these hearing impairments, the cochlea may function perfectly and a bone conduction hearing device can transmit the sound signal more efficiently to the cochlea. Today, the percutaneous bone anchored hearing aid (BAHA) is an important alternative for such individuals. This device uses a percutaneous snap coupling together with a bone anchored implant to transmit the sound vibrations to the skull bone and has proven to offer very good hearing rehabilitation. However, such a system with permanent skin penetration requires a life-long commitment of care every day, may cause skin infections, and there is a risk for implant damage due to trauma and hence improvements are called for in these aspects. A novel bone conduction implant (BCI) device is designed as an alternative to the percutaneous BAHA device, because it leaves the skin intact. The BCI device provides a specific hearing aid digital signal processor, and analog signal processing parts. By applying amplitude modulation technique, the sound signal is transmitted to a permanently implanted transducer via an inductive link system through the intact skin. An efficient wireless power and data transmission system for the BCI device has been designed and implemented. Maximum power output (MPO) of the BCI was designed to occur at 4 mm skin thickness. The power output variability for 1 to 8 mm skin thickness variations was within 1.5 dB. Maximum MPO was found to be 109 dB relative to 1 μN at transducer resonance frequency. This implant system consumes 14.6 mA of battery current at 1 kHz at 65 dB input sound pressure level. It was also found that the gain headroom improvement with the BCI versus the BAHA was in the range of 10-30 dB, if the mechanical output of the devices were normalized at the cochlear level.